Parameter `plot-background-alpha` has no effect (since 8.1)

[bksaiki]

It seems that plot-background-alpha no longer works when generating png files with plot-file. This bug was introduced after Racket 8.0. Here's a quick example:

(require plot/no-gui)

(define out (open-output-file "img.png" #:exists 'replace))

(parameterize ([plot-width 800] [plot-height 300]
               [plot-background-alpha 0])
  (define xs (build-list 20 (λ _ (random))))
  (define ys (build-list 20 (λ _ (random))))
  (define pts (points (map vector xs ys)
                      #:color "black"))
  (plot-file (list pts) out 'png
             #:x-min 0 #:x-max 1
             #:y-min 0 #:y-max 1))

Please let me know if something has changed with this parameter.

[alex-hhh]

Thanks for reporting this. This was likely introduced by the plot-metrics changes in #90. A call to (make-bitmap ...) was replaced with a call to (make-object bitmap% ...) but the two calls have different defaults for whether to create an alpha channel or not. The (make-object bitmap% ...) variant defaults to not having an alpha channel, introducing this bug.

The workaround for now is to create the bitmap separately and use plot/dc:

#lang racket
(require plot/no-gui racket/draw)

(define out (open-output-file "img.png" #:exists 'replace))
(define bm (make-bitmap 800 300))
(define dc (send bm make-dc))

(parameterize ([plot-width 800] [plot-height 300]
               [plot-background-alpha 0])
  (define xs (build-list 20 (λ _ (random))))
  (define ys (build-list 20 (λ _ (random))))
  (define pts (points (map vector xs ys)
                      #:color "black"))
  (plot/dc (list pts)
           dc 0 0 800 300
           #:x-min 0 #:x-max 1
           #:y-min 0 #:y-max 1))

(send bm save-file out 'png)

The fix is to specify that an alpha channel needs to be created in these places:

plot/plot-lib/plot/private/no-gui/plot2d.rkt

Line 107 in c412600

(make-object (plot-metrics-mixin bitmap%) (λ () pm) width height))

plot/plot-lib/plot/private/no-gui/plot3d.rkt

Line 138 in c412600

(make-object (plot-metrics-mixin (class bitmap% (super-new))) (λ () pm) width height))

[bksaiki]

Thanks @alex-hhh. I'll use the workaround until a fix gets merged.

[alex-hhh]

Racket snapshot builds now contain a fix for this issue and the fix will also be available in Racket 8.2